Polymer dispersed liquid crystal (PDLC) materials have been shown useful for the construction of displays and other electrooptic (EO) devices, as pointed out by Drzaic in Liquid Crystal Dispersions (World Scientific Publishing Co., Singapore, 1995). Further, for example, Doane et al. in U.S. Pat. No. 4,688,900 describe the construction and operation of scattering mode devices based on PDLC films. Those films are comprised of nematic liquid crystal droplets dispersed in a polymer matrix. The resulting devices exhibit an optical response from a highly scattering state to a substantially clear state in response either to an electric field or by thermal means, through control of the PDLC film temperature. While such films are useful for displays or scattering mode polarizers, they are unsuitable for use in diffractive or non-dispersive optical devices since they exhibit high scattering loss in the visible and near infrared. In addition, the switching time of such devices is generally slow, on the order of 1–50 ms.
Sutherland et al. in U.S. Pat. No. 5,942,157 describe how PDLC materials may be formulated and applied to the fabrication of diffractive and refractive EO elements. In that patent, based upon the materials used, the description points out that devices made with materials comprising above 35% liquid crystal content are highly scattering, and therefore are undesirable for EO devices. In addition, such formulations yield materials with very low values of index modulation. For example, Sutherland et al. report in Chemical Materials 5, 1533–38 (1993) that an index modulation of less than 0.003 was measured in transmission gratings made using PDLC materials with low liquid crystal concentration. Consequently, high contrast gratings that operate in the visible could only be realized in extremely thick gratings, on the order of 50 μm thick. PDLC gratings this thick are impractical for use in both the visible and infrared because they require an extremely high switching voltage, and further, because they are inherently lossy due to scattering.
It is therefore an object of this invention to utilize PDLC materials for diffractive and refractive optical components that exhibit extremely low insertion loss.
It is another object of this invention to obtain high index of refraction modulation in photo-curable PDLC films by curing them with a spatially inhomogeneous illumination source. Such films enable the construction of thin, high efficiency, and low-drive-voltage devices.
It is yet another object of this invention to fabricate PDLC-based EO diffractive and refractive devices that display switching times in the microsecond regime.